1. Field of the Invention
The present invention relates to an endoscope system for performing narrowband light observation using a complementary color type imaging device, and a light source device used in the endoscope system.
2. Description Related to the Prior Art
In a recent medical field, diagnosis and treatment using an endoscope system, having a light source device, an electronic endoscope, and a processor device, are widely performed. The light source device produces illumination light and applies the illumination light to the inside of a human body cavity. The electronic endoscope images the inside of the body cavity irradiated with the illumination light by an imaging device, and produces an imaging signal. The processor device applies image processing to the imaging signal produced by the electronic endoscope to produce an observation image to be displayed on a monitor.
As an observation method used in the endoscope system, there is known narrowband light observation using special light (narrowband light) having a narrow wavelength band as the illumination light, in addition to normal light observation using normal light (white light) having a wide wavelength band as the illumination light. The narrowband light observation, for example, can improve visibility of a blood vessel pattern in a superficial layer of a mucosa membrane, though the blood vessel pattern is easily buried in optical information obtained under irradiation with the white light. Therefore, the narrowband light observation allows focusing attention on superficial blood vessels of the blood vessel pattern, and diagnosing the stage of a disease, the depth of a lesion, and the like from the state of the superficial blood vessels.
The narrowband light observation uses two types of narrowband light absorbable by hemoglobin in blood, that is, blue narrowband light having a center wavelength in the vicinity of 415 nm and green narrowband light having a center wavelength in the vicinity of 540 nm. As an imaging method in the narrowband light observation, there are known a frame sequential method in which the blue narrowband light and the green narrowband light are alternately applied and a monochrome imaging device captures an image whenever each type of light is applied, and a simultaneous method in which the blue narrowband light and the green narrowband light are simultaneously applied and a simultaneous imaging device having color filters captures an image (see U.S. Pat. No. 8,531,512 and US Patent Application Publication No. 2009/0141125). The simultaneous method is inferior in resolution to the frame sequential method, but has the advantages of preventing a blur in the image and structural simplicity of the endoscope system.
The simultaneous imaging device includes a primary color type imaging device having primary color filters and a complementary color type imaging device having complementary color filters. The primary color type imaging device is used in an endoscope system that places importance on color, because of being superior in color reproducibility, though inferior in sensitivity, to the complementary color type imaging device. On the other hand, the complementary color type imaging device, which is superior in sensitivity and inferior in color reproducibility to the primary color type imaging device, is used in an endoscope system that places importance on sensitivity. Since the primary color type imaging device and the complementary color type imaging device have both advantage and disadvantage, it is desired that an endoscope system of the future be available with both of a primary color type endoscope containing the primary color type imaging device and a complementary color type endoscope containing the complementary color type imaging device.
The U.S. Pat. No. 8,531,512 and the US Patent Application Publication No. 2009/0141125 disclose a complementary color type imaging device of a complementary-color checkered-pattern color-difference line sequential method having four types of pixels of magenta (Mg), green (G), cyan (Cy), and yellow (Ye). According to the complementary-color checkered-pattern color-difference line sequential method, pixel signals are read out by a field readout method in a state of mixing (adding) the pixel signals of two adjoining rows. More specifically, the pixel signals are read out in a state of four types of combinations, i.e. the Mg pixel and the Cy pixel, the G pixel and the Ye pixel, the Mg pixel and the Ye pixel, and the G pixel and the Cy pixel. The complementary-color checkered-pattern color-difference line sequential method has the advantage of ease of producing a Y/C signal and an RGB signal just by addition and subtraction of the signals of the four types of mixed pixels.
In the case of performing the narrowband light observation by the endoscope system described above, according to the primary color type imaging device, blue (B) pixels capture the blue narrowband light and green (G) pixels capture the green narrowband light, independently. Thus, the primary color type imaging device can produce an image that has high color separability and high visibility of the superficial blood vessels (high contrast between the superficial blood vessels and the mucosa membrane). On the contrary, in the complementary color type imaging device, each mixed pixel senses the blue narrowband light and the green narrowband light at the same time (i.e. mixture of colors occurs). This causes low color separability, and a blur of the superficial blood vessels due to the influence of scattered light deteriorates the visibility of the superficial blood vessels.
In relation to this problem, the U.S. Pat. No. 8,531,512 and the US Patent Application Publication No. 2009/0141125 describe that changing coefficients of a matrix operation for converting a Y/C signal into an RGB signal in accordance with characteristics and the like of color filters reduces color mixture. However, this matrix operation is performed in a signal processing circuit that performs an operation of mixed pixel signals, and each of the mixed pixel signals is a signal in which a blue narrowband light component and a green narrowband light component have already been mixed at the time of entering the signal processing circuit. Therefore, the matrix operation cannot radically improve the color separability and the visibility of the superficial blood vessels.